1. Field of the Invention
The present invention generally relates to semiconductor devices, and particularly relates to a semiconductor device which is provided with a voltage-level-detection circuit for detecting a voltage level of an internal voltage.
2. Description of the Related Art
Semiconductor devices are generally equipped with a mechanism to detect a level of a voltage used inside the devices. For example, when a voltage is internally generated in a semiconductor device for internal use, this internally generated voltage, which is different from a power voltage VDD and a ground voltage VSS externally provided, needs to be checked as to whether this voltage is kept at a predetermined voltage level. Based on the check results, the voltage level of the internally generated voltage is controlled.
In semiconductor memory devices such as DRAMs, when a voltage level VDD is to be stored in a capacitor serving as a memory cell, a cell transistor connected to this memory cell is turned on so as to supply electric charge from a bit line to the memory cell. In this case, the cell transistor needs to receive as a gate input thereof a voltage which is higher than the target voltage level VDD by a margin of a threshold voltage of the transistor. Moreover, this gate voltage needs to be further higher by a margin of an overdriving voltage in order to achieve high-speed charging up of the memory cell. When such a voltage level, higher than the externally provided power voltage, is required, a voltage pumping circuit is used. The semiconductor memory device may malfunction, however, when the pumped voltage declines as current consumption inside the device increases out of proportion to the supply of the pumped voltage. In order to avoid this, the pumped voltage level needs to be detected to attend to voltage-level control based on the detected voltage level.
Similar voltage-level detection and voltage-level control should also be conducted with regard to a substrate voltage, which is set to a voltage lower than the ground voltage in semiconductor devices.
The voltage-level-detection circuit used for above-described voltage-level detection detects a voltage level by dividing a target voltage using a series of resistors and by comparing the divided voltage with a reference voltage. Such a voltage-level-detection circuit is in operation at all the time, so that the series of resistors keeps consuming electric power.
This electric-power consumption is small. In view of recent development in the reduction of power consumption in semiconductor devices, however, even a small power consumption cannot be ignored.
Accordingly, there is a need for reducing power consumption in a voltage-level-detection circuit of a semiconductor device.